The first order parameters which normally limit the maximum length of the communication link are
(1) magnitude of coupled power,
(2) fiber losses, and
(3) receiver sensitivity.
In general, the only control a designer has over the first two parameters is by component and fiber selection. In the case of receiver design, however, circuit design expertise can yield up to 10 dB (optical) improvement over conventional approaches which can add several kilometers to the maximum distance achievable without repeaters. With this in mind, one of the areas of improvement which can be made concerns the use of the pre-amplifier section.
In co-pending application Ser. No. 149,291, filed by Paul W. Casper et al on May 12, 1980 entitled "multi-Channel, Repeatered, Fiber Optic Communication Network" and assigned to the assignee of the present application, there is described a system in which optical signals are transmitted over the fiber optic communication links and received by an optical receiver module. In the receiver module the incoming signals are converted by means of an avalanche photo diode and are made ready for further processing through the use of a pre-amplifier. Advantageously, the system described in the above-referenced application employs a transimpedance pre-amplifier according to the present invention. Although the transimpedance amplifier of the present invention applies specifically to the above-identified co-pending application, it also applies to any high data rate optical signal system such as the T4 system.
The problem of obtaining low distortion and flat frequency response over a wide range of input signals is solved in the prior art by the use of negative feedback. When the input signal is a current as in the case for avalanche photo diodes and other diode optical detectors, a "transimpedance amplifier" (an operational amplifier with a single resistor for the feedback network) is an obvious choice. However, in the conventional implementation, the feedback resistor-input capacitance-system bandwidth product must be kept small because of the additional phase shift caused by the feedback resistance and input capacitance. This results in the amplifier being dominated by the thermal noise of the feedback resistor. Therefore, one of the prior art problems which must be solved is the implementation of a feedback system of a transimpedance amplifier which can produce low noise, low distortion and flat frequency response in the presence of a large value of feedback resistor-input capacitance-system bandwidth product.